Has your DH really got zero savings?  If that is the case, he shouldn't have "promised" his son that his car parts would be financed, without discussing it with you first.  I suppose you could make DH a loan of the $3k, and he pays you back over the next few weeks or months, as he can afford.  I really, really don't see that you should be out of pocket over this. Like you, I never asked my parents for money, ever. I would never even consider giving the SDs money that was mine (DH and I have separate finances).  

DH and I agree he pays me back when he "generously" agrees to bail one of his kids out.  And, I collect in full tho I can handle monthly payments from him.  DH would probably prefer that I be the magic fairy godmother and sprinkle money on everybody but I'm not.  This works for us.

I would NOT pay for anything for a grown man who has been sitting on has azz for over a year, not working. The job market is not the best in my area but there are still plenty of businesses hiring at the $12-$15 an hour. When one needs money, one does what one can do, including flipping burgers. Ask me how I know. 

You say you guys are retired. So are DH and I. We recognize, while we are comfortable and do well, our finances ARE LIMITED. We are both healthy and plan to live long, if the good Lord allows. We simply must be careful for our money. DH had one of his kids reach out for a "loan" a while back. $2000 because she got evicted. Able bodied woman who CAN work. DH declined, explained we are retired and have limited funds now. I suggest your DH call his son and tell him the same. Apologize for makiing an offer HE cannot fulfill. SMH

Best to you.

I wouldn't contribute personal savings to SS. I would leave it up to your husband if he wanted to help but preferably he wouldn't. The idea being that if you both decline help it will make SS a bit more uncomfortable which might possibly put a fire under his butt to apply to better jobs and set himself up financially. If he's bailed out he won't have incentive to try to handle things like this on his own moving forward. He needs to become self reliant. 100 percent I wouldn't dip into personal savings to help. 

You are definitely Not the Ahole. If anything, DH should be asking himself that question due to putting the idea in his adult son's head that money ( yours) was coming to him wihout  bothering to ask you first. That would be a hard NO from be just based on that.  Your SS is just going to have to "rough it" by considering those jobs under 6 figures that he feels he is "too good " for instead of sitting around not contributing at all. Or at least start living within his means.

I woudnt give him a dime in that case.

NOPE DO NOT give ss money.

Here is the thing, you wrote "I flat out told DH that if I agree to this it's for him, not for his son, who prefers that I not exist. I guess that was a step too far. "

Why are you doing it for DH--Your ss is an ass, he has not worked in 1 year and your dh has allowed his adult grifter to treat you like garbage.

SCREW that. ---Let them figure it out.

I just dont understand why this is a question. 

Yeah, second everything Thumper has just posted.

I'll add that if you "do this for DH" this time there will be a next time, and a next time...

What kind of parts are these

.. cosmetic or so he can drive his only vehicle? Obv not your responsibility but I agree it would be considered a favor to dh

If the parts are necessary for the car to drive, and thus work (if he's truly trying to), can you lend the money to SS with the stipulation that HE pays you back? Let him know that DH can't help him but you can. Or, lend it to DH and DH has to take the payback money out of his discretionary funds. Such as he has to "save" it somehow by doing without a luxury. Idk, maybe that is too much. I lent my SO 3k for lawyer bills and he said he would pay me back. After about a month, i saw SO looking at some unnecessary luxury items saying he might buy them and i said "You still haven't paid me back. It's time." ETA you are also perfectly within your rights to say no. He is a long-term unemployed adult man. He should have taken a job "beneath" him after a few months. 

Honestly there really is no excuse for not working at this point. Even if he's getting unemployment, it's not more than even the lower-paying jobs he could get. He could be Ubering. He could definitely be making enough $ to pay for these car parts himself. I would NOT give him this $. He is not working because he doesn't want to. It's your money and your DH is trying to enable him. I myself am not one for enabling, especially when I'm the one that's going to lose out. Time for a tough lesson SS, which as an adult you should already know. 

I would not give SS a dime! Especially if he treats you poorly. That is telling him that he can treat you poorly and he still cashes in. Is that the message you wish to convey?

Nope. definitely NTA.

Time for daddy to bare his own ass to his spawn and renege on his naive immature promise to SS.

Parents should never jeopardize their own financial security for Kidult spawn.

We are fortunate that our kid (my former SS-32 who I adopted at his request 10yrs ago) is fully self supporting and well on his way to a secure retirement.

As for SS and his car parts.  He can shop and go with LKQ parts Vs new.

I recommend LKQ-Keystone for him to look into. They do Like Kind & Quality (LKQ) parts.  Far less expensive than new.  I just had work done on my vehicle after hitting a curb and damaging a wheel and the suspension of my vehicle. My vehicle is a 2018 model and we are not going to buy new vehicles for another 5 or so years.  Our insurance covers for LKQ replacement parts for anything older than 5 years.  So, they replaced the damaged wheel with an LKQ wheel for $400 compared to $1300 for a new OEM replacement.  LKQ suspension parts were $40 Vs another $2K for new OEM parts.  My insurance covers the parts for as long as I own the vehicle though the parts are not brand new.  Labor of course is a different story.  Labor for the repairs was $3K which your SS can avoid if he is doing the repairs himself.  So, LKQ should be the direction SS goes IMHO.

He can figure out how to pay for it.  If you live in a city with a LKQ-Keystone location, he may be able to just pay cash unless they have to source the parts and ship them in. They will also ship them directly to you.   == If there is no LKQ-Keystone location in your area and SS has no credit car that he can use to oerder the parts from LKG-Keystone,  SS needs to hit the salvage yards, pull the parts himself, and pay the yard cash.  Most yards these days will pull the parts for you for a fee.

Time for Skippy to figure it out without begging daddy for $Thousands.

IMHO of course.

That tells me DH is no good with money. He will spend his and yours..  He should take out a loan if he wants to give SS  1,000 S OF $.  3,000 is a good start.  Just make sure that DH understands him paying off that loan doesn't mean you will be making up for his short fall.'s in money.   
'This seams like a life long money problem's  in his family.  Not your problem 

The answer is: NO. Don't give your hard earned savings to this squatter who treats you like garbage. Seriously, uncomplicate the situation that will turn into a never-ending revolving door with SS then SD, etc... Hold strong, don't bend for these people. You don't owe them a thing. 

Is his car worth a 3K repair job? I have almost always, at least until retirement, driven a used car and quickly learned that when one big thing happened to my car others were not far behind.

if you could tie the car repair $$$ to his getting a job. Plus the conversation about loan terms, of course.